1. Field of the Invention
The present invention relates to a fixing device. More particularly, the present invention relates to a fixing device arranged in a photolithographic image forming apparatus such as a laser printer, a laser facsimile, or a digital copier.
2. Description of the Related Art
In electrophotographic image forming apparatuses, the surface of a photoconductor drum is charged and then scanned by a laser scanning unit or the like to form an electrostatic latent image according to an image signal. Thereafter, charged toner is attached onto the photoconductor drum and developed by a developer to form a toner image. Then, the toner image is transferred to a paper sheet (recording medium). The toner image transferred to the paper sheet is seated on the sheet, that is, not fixed thereto, and the toner image is then fused with heat and pressure to fix the paper sheet by a fixing device.
A conventional fixing device basically includes a cylindrical metallic heat roller (an electrical heating element), a press roller (a pressing element) which presses and contacts the heat roller, and a halogen lamp (a heat source) installed in the heat roller to heat the heat roller with radiant heat. In the fixing device (2-roller type), the paper sheet having a toner image formed thereon is passed through a fixing nip formed between the heat roller and the press roller, which contact with each other with pressure, so that the toner image is heated by the heat roller and pressed on and fixed to the sheet by the pressure contact of the heat roller and the press roller.
However, it takes a rather long period of time for a conventional fixing device to heat the heat roller at room temperature to reach a fixing temperature of about 160 to 200° C. at which toner melts. Hence, a conventional fixing device spends a significantly long warm-up time before actual printing after power application. Furthermore, the fixing device maintains its temperature high because this shortens the time interval between print standby and print start. The fixing device needs to be heated with a halogen lamp to keep its temperature high. This results in increased power consumption.
To overcome these disadvantages of the conventional 2-roller type fixing device there have been recent proposals for fixing devices capable of reducing both warm-up time and power consumption.
One of the proposed fixing devices is an induction heating (IH) fixing device which achieves heat generation by inducing current using magnetic flux. The IH fixing device basically includes a cylindrical or half-cylindrical holder, a cylindrical film fits into the holder comprising a stack of conductive and resistor layers, a magnetic field generating unit installed in the holder and comprised of an excitation coil and a magnetic core, and a press roller that presses and contacts an outer circumference of the film to form a fixing nip. In the IH fixing device, an Eddy current is generated by the magnetic flux generated by the magnetic field generating unit. As a result, Joule's heat is generated on the conductive layer by the Eddy current, and the temperature of the fixing nip is increased by the Joule's heat. Due to the IH, the temperature of the fixing nip can be increased to a fixing temperature within a short time period, and the warm-up time period can be reduced. Furthermore, as described in Japanese Patent Publication No. 2004-126274, the entire disclosure of which is hereby incorporated by reference, power consumption can be reduced because the cylindrical film has a low heat capacity compared to a heat roller. However, because the IH fixing device needs the magnetic field generating unit, it requires high costs for components compared to the 2-roller type fixing device. Moreover, because the inner surface of the film slides on a component such as the holder, the durability of the film is restricted due to abrasion of the holder portion on which the film slides. Therefore, the selection range of a material for the film is relatively narrow.
There is also a press belt pressing type fixing device in which a tube-shaped press belt is included instead of the press roller included in the 2-roller type fixing device. A fixing nip is formed by pressing a part of the inner surface of the press belt toward a heat roller so that the outer surface of the press belt corresponding to the pressed inner surface portion contacts the heat roller. The press belt pressing type fixing device basically includes a heat roller which is heated by a heat source such as a halogen lamp and axially rotates, an endless press belt which contacts the heat roller and is moved by a motion of the heat roller, and a pressing element which presses an inner surface of the press belt toward the surface of the heat roller so that an outer surface of the press belt corresponding to the pressed inner surface contacts the heat roller. As described in Japanese Patent Publication No. 2004-12682, the entire disclosure of which is hereby incorporated by reference, the fixing nip is wide because it is formed by pressing the press belt on the heat roller so that they contact each other. Accordingly, the warm-up time and power consumption can be reduced, and component costs can be reduced from those required by the IH fixing device. However, because the inner surface of the press belt is pressed by the pressing element and slides on the heat roller, the durability of the press belt or the pressing element is still restricted due to abrasion of the heat roller on which the press belt slides, and the selection range of a material for the press belt or the pressing element is still narrow.
Additionally, a heat belt type fixing device includes a cylindrical heat belt unit comprising an endless heat belt which is freely movable in a main direction is included instead of the heat roller included in the 2-roller type fixing device. A fixing nip is formed by contacting the heat belt with a cylindrical press roller with pressure. The heat belt type fixing device basically includes the heat belt unit manufactured by winding the endless heat belt on a support roller and the cylindrical heat roller. The cylindrical press roller presses a bridging portion of the heat belt ranging between the support roller and the heat roller to contact the bridging portion. A halogen lamp is installed in the heat roller or the press roller. In the heat belt type fixing device, a wide fixing nip is formed, so that the warm-up time and power consumption can be reduced. Furthermore, because no sliding occurs, no abrasion occurs. Therefore, numerous materials for the heat belt can be used. In addition, as described in Japanese Patent Publication No. 2004-205877, the entire disclosure of which is hereby incorporated by reference, component costs can be less than those for the IH fixing device.
However, because the heat belt in the heat belt type fixing device is supported by the support roller, and the heat roller and is tensed, when the heat belt is rotated it moves toward the shaft of the support roller. Alternatively, the heat roller may tilt due to an error due to size accuracy, assembly accuracy of each roller, or a widthwise deviation of the circumference of the heat belt.
Since a color toner image is formed by stacking toner layers of a plurality of colors, the image may be difficult to fuse. Thus, an offset phenomenon may occur where the toner image partially separates. In the above-described conventional fixing devices, when the heat belt or heat roller having residual toner attached thereto rotates and contacts the sheet, the residual toner attaches to the sheet, resulting in image degradation.
Accordingly, there is a need for an improved image forming apparatus including a fixing device which can reduce warm-up time, power consumption, and tilting of a belt to improve image quality.